This invention relates to electrophotography. More particularly, it relates to polymers comprising a tetracarbonylbisimide group and to photoconductive elements that contain an electrical charge barrier layer comprised of said polymers.
Photoconductive elements useful, for example, in electrophotographic copiers and printers are composed of a conducting support having a photoconductive layer that is insulating in the dark but becomes conductive upon exposure to actinic radiation. To form images, the surface of the element is electrostatically and uniformly charged in the dark and then exposed to a pattern of actinic radiation. In areas where the photoconductive layer is irradiated, mobile charge carriers are generated which migrate to the surface and dissipate the surface charge. This leaves in non-irradiated areas a charge pattern known as a latent electrostatic image. The latent image can be developed, either on the surface on which it is formed or on another surface to which it is transferred, by application of a liquid or dry developer containing finely divided charged toner particles.
Photoconductive elements can comprise single or multiple active layers. Those with multiple active layers (also called multi-active elements) have at least one charge-generation layer and at least one n-type or p-type charge-transport layer. Under actinic radiation, the charge-generation layer generates mobile charge carriers and the charge-transport layer facilitates migration of the charge carriers to the surface of the element, where they dissipate the uniform electrostatic charge and form the latent electrostatic image.
Also useful in photoconductive elements are charge barrier layers, which are formed between the conductive layer and the charge generation layer to restrict undesired injection of charge carriers from the conductive layer. Various polymers are known for use in barrier layers of photoconductive elements. For example, Hung, U.S. Pat. No. 5,128,226, discloses a photoconductor element having an n-type charge transport layer and a barrier layer, the latter comprising a particular vinyl copolymer. Steklenski, et al. U.S. Pat. No. 4,082,551, refers to Trevoy U.S. Pat. No. 3,428,451, as disclosing a two-layer system that includes cellulose nitrate as an electrical barrier. Bugner et al. U.S. Pat. No. 5,681,677, discloses photoconductive elements having a barrier layer comprising certain polyester ionomers. Pavlisko et al, U.S. Pat. No. 4,971,873, discloses solvent-soluble polyimides as polymeric binders for photoconductor element layers, including charge transport layers and barrier layers.
The known barrier layer materials have certain drawbacks, especially when used with negatively charged elements having p-type charge transport layers. Thus, a negative surface charge on the photoconductive element requires the barrier material to provide a high-energy barrier to the injection of positive charges (also known as holes) and to transport electrons under an applied electric field. Many known barrier layer materials are not sufficiently resistant to the injection of positive charges from the conductive support of the photoconductive element. Also, for many known barrier materials the mechanism of charge transport is ionic. This property allows for a relatively thick barrier layer for previously known barrier materials, and provides acceptable electrical properties at moderate to high relative humidity (RH) levels. Ambient humidity affects the water content of the barrier material and, hence, its ionic charge transport mechanism. Thus, at low RH levels the ability to transport charge in such materials decreases and negatively impacts film electrical properties. A need exists for charge barrier materials that transport charge by electronic as well as ionic mechanisms so that films are not substantially affected by humidity changes.
Still further, a number of known barrier layer materials function satisfactorily only when coated in thin layers. As a consequence, irregularities in the coating surface, such as bumps or skips, can alter the electric field across the surface. This in turn can cause irregularities in the quality of images produced with the photoconductive element. One such image defect is caused by dielectric breakdowns due to film surface irregularities and/or non-uniform thickness. This defect is observed as toner density in non-image areas, also known as background density.
Photoconductive elements typically are multi-layered structures wherein each layer, when it is coated or otherwise formed on a substrate, needs to have structural integrity and desirably a capacity to resist attack when a subsequent layer is coated on top of it or otherwise formed thereon. Such layers are typically solvent coated using a solution with a desired coating material dissolved or dispersed therein. This method requires that each layer of the element, as such layer is formed, should be capable of resisting attack by the coating solvent employed in the next coating step.
Accordingly, a need exists for a negatively chargeable photoconductive element having a p-type photoconductor, and including an electrical barrier layer that can be coated from an aqueous or organic medium, that has good resistance to the injection of positive charges, can be sufficiently thick that minor surface irregularities do not substantially alter the field strength, and resists hole transport over a wide humidity range. Still further, a need exists for photoconductive elements wherein the barrier layer is substantially impervious to, or insoluble in, solvents used for coating other layers, e.g., charge generation layers, over the barrier layer.
Photoconductive elements comprising a photoconductive layer formed on a conductive support such as a film, belt or drum, with or without other layers such as a barrier layer, are also referred to herein, for brevity, as photoconductors.
The above objects and advantages can be attained by the present invention, which relates to photoconductive elements comprising an electrically conductive support, an electrical barrier layer and, disposed over the barrier layer, a charge generation layer capable of generating positive charge carriers when exposed to actinic radiation. The electrical barrier layer, which restrains injection of positive charge carriers from the conductive support, comprises a condensation polymer having as a repeating unit a planar, electron-deficient, tetracarbonylbisimide group. Without wishing to be bound by theory, it is believed that such bisimide group can transport charge primarily by electronic rather than ionic transport mechanisms.
In addition to the tetracarbonylbisimide group, in embodiments, the condensation polymer further comprises an ionic moiety, such as the polyesterionomer-co-imides described more fully hereinbelow and in our parent application, U.S. Ser. No. 09/574,775, filed May 19, 2000. While not wishing to be bound by theory, it is believed that the ionic moiety is capable of imparting ionic transport characteristics to the condensation polymer. Thus, by combining the tetracarbonylbisimide group with an ionic moiety, it is believed that the resulting condensation polymer is capable of transporting charge by both ionic and electronic mechanisms and thereby provides acceptable performance across a wider range of RH levels as shown in the illustrative examples that follow hereinafter. In some preferred embodiments which incorporate said ionic moiety, as more fully described hereinafter, the condensation polymer can include an ether repeat unit in the polymer backbone, which unit can impart desirable mechanical and/or electrical properties into the electrical barrier layer.
More specifically, in the photoconductive element of the invention, said barrier layer comprises a condensation polymer having covalently bonded as repeating units in the polymer chain, aromatic tetracarbonylbisimide groups of the formula: 
wherein Ar1 and Ar2 represent, respectively, tetravalent or trivalent aromatic groups of 6 to about 20 carbon atoms and X is O, C(CF3)2, Sxe2x95x90O or SO2.
More specifically, in embodiments, the barrier layer polymer is a polyester-co-imide, polyesterionomer-co-imide, or polyamide-co-imide that contains an aromatic tetracarbonylbisimide group, and has the formula: 
wherein:
Q represents one or more groups selected from:
(a) an alkylenedioxy, aromatic dicarboxyl, and aromatic diamino groups having from about 2 to about 36 carbon atoms; 
Ar3, Ar4, and Ar7 independently represent a tetravalent aromatic group having from about 6 to about 20 carbon atoms;
Ar5 and Ar6 independently represent a trivalent aromatic group having from about 6 to about 12 carbon atoms;
R1, R2, R3, R4, R5, and R6 independently represent alkylene or alkyleneoxy groups having from about 2 to about 12 carbon atoms;
L1, L2, L3, L4, L5, and L6 independently represent O, CO, CO2, or NH;
Z1 and Z2 independently represent an alkylenedioxy or alkylenediamino group having from about 2 to about 36 carbon atoms;
X is O, C(CF3)2, Sxe2x95x90O or SO2; and
x and y represent mole fractions, x being the mole fraction of the group that contains Ar3 and y being the mole fraction of the group, Z1; and wherein x is from about 0.05 to 1 and y is from 0 to about 0.95.
In other embodiments, the photoconductive element comprises an electrically conductive support, an electrical barrier layer and, disposed over the barrier layer, a charge generation layer capable of generating positive charge carriers when exposed to actinic radiation. The barrier layer comprises a polyesterionomer-co-imide condensation polymer, which includes as a repeating unit, a planar, electron-deficient, aromatic tetracarbonylbisimide group.
In some preferred embodiments, the barrier layer polymer is a polyesterionomer-co-imide represented by the formula: 
wherein:
Ar8 is a tetravalent aromatic group having from about 6 to about 20 carbon atoms;
Q1 represents an alkyleneoxy group having from about 2 to about 36 carbon atoms;
R7 and R8 independently represent alkylene or alkyleneoxy groups having from about 2 to about 12 carbon atoms;
L7 represents an aromatic moiety having from about 6 to about 20 carbon atoms;
L8 represents a saturated or unsaturated, linear, branched, and or cyclic aliphatic group having from about 2 to about 24 carbon atoms;
Axe2x88x92 represents an ionic moiety selected from sulfonates, phosphonates, sulfonamides, or bissulfonamides;
M+ represents a counterion selected from alkali metal, ammonium, or phosphonium cations;
Y is a mole fraction having a value of from about 0.05 to 1;
X is a mole fraction having a value of from about 0.2 to about 0.8; and
N is a mole fraction having a value of from 0 to about 0.8.
The barrier layer polymers described above are also preferably substantially insoluble in solvents used for coating the charge generation layer over the electrical barrier layer under the coating conditions employed. The preferred polyesterionomer-co-imides described above can also generally resist both swelling and solubilization during the time frame for the coating step associated with formation of the charge generation layer.